Need replacement for BC847S better suited for HF

Hello Erik,

Check Toshiba's transistor pairs. IIRC Digikey has them.

Else, how about using a couple RF transistors and glue their bodies together for better thermal coupling? Just keep in mind that their beta will usually be quite low and so will their breakdown voltage.

Regards, Joerg

Hi Joerg,

thanks for you hint. I checked toshibas transistor pairs: Regarding their parameters they would be very fine but none of them has the pinout I need:

BC847:

C1 B2 E2

Toshiba's transistors all have the same pinout:

B1 B2 E2

Size of such a dual transistor is below 2mm and I have got three of them in 1.5cm^2. I think applying such a patch would be really challenging, too. But maybe it is worth a try before redesigning the PCB...

What would be your choice for the transistor? Maybe in SOT23 package with teh following pinout?

• !\ ! \ ! \

Thats OK. The total gain of the stages is below 5 and breakdown voltage of 5V would be fine.

Thanks for your hints,

best regards,

Erik.

Hello Erik,

In that case maybe one of my old favorites, the BFS17. But your desired pin-out is non-standard. There used to be a reversed version called BFS17-R. I don't know why they came out with it, maybe because in the early days lots of SMT layouts went wrong (and they still do...). I haven't seen "-R" versions in a long time. So you may have to take a regular one and carefully bend the leads backwards. Or use a non-SMT transistor like the 2N2369.

But keep in mind, these are hot rods. Transistors with an fT in the GHz range can easily become oscillators. Thing is, there ain't much around in the "medium speed" range. Most popular transistors are either below

Regards, Joerg

OK, thanks a lot. I will order some BFR17 and give it a try! I will keep you up to date!

Interested??!?!?!!! Around here, that's _required_! ;-)

Cheers! Rich

Hello Erik,

It's BFS17, not BFR17.

Regards, Joerg

You will find the entire documentation (including the schematics and PCB layout) on the following link:

Best regards,

Erik.

Of course - it was only a typo! Thanks!

Thanks for the Tip - I patched one of my boards with two BFS17 instead of one BC847S. Additionally I replaced the 1N4148 with a half BAT68-06 which has much lower capacity. Analysis with the network analyzer shows that it is now nearly as good as the original 04KH009:

On

My plan now is to redesign the board for using transistors from the BFS480/481/482 series since they are readily available. As well I will put in the BAT68-6 since the board can be even smaller than allowing me to apply some kind of shielding...

Thanks again,

Erik.

BTW: Applying the patches (i.e. 8 BFS17 transistors) was real pain since SOT363 is very tiny and SOT23 is still small - but huge compared to SOT363 ;-)

Hello Erik,

Looking pretty good there but you have some droop towards 500MHz. You might need an even hotter transistor with a higher fT. Since you are in Germany I'd look into Infineon. In the US I found it nearly impossible to obtain samples so I don't use them anymore but that might be easier in Europe. Right now they data sheet link don't work (happens a lot with them, they seem to use some stupid java script...) but check the BFR series. There are some above 10GHz fT. And really, really small.

Regards, Joerg

I think the BFS480 are "hotter" than the BFS17 - at least a factor of two. The oscilloscope under investigation is a C1-122 and the other componentes have -3dB at around 150MHz so a drop at 750MHz (as I expect in using the BFS48x) might be OK. In my opinon the SOT363 package is the smallest I want to solder by hand ;-)

If their site works again I will be happy to check out the BFR series - maybe I will make two different PCBs to give them a try, too...

Regards,

Erik.

The Russian stuff is probably using an inferior and corrupt design to begin with. Rip their dumbass chopper and replace with cheap GaAsFET logic controlled switches and possibly a video summing amplifier if you need gain.

The design is old (maybe between 1970 and 1980) but I'd not say that it is corrupt. It was state of the art this time and is found in contemporary tek oscilloscopes as well.

There is no gain needed in this application but the impedance of the chopper is of importance since directly after the chopper there is the circuitry responsible for bandwidth selection. Using CMOS-Switches would be difficult, too: The 04KH009 is designed fo differential signals and shifts the level of both lines by several volts from input to output side. I.e. replacing the 04KH009 by any type of modern switch would require severe patches to the vertical amplifier plugin I think. But the idea of using a video summing amp for such purposes is a interesting hint one should keep in mind. Do you have a type number, so I can look at a datasheet?

Thanks for your ideas,

Erik.

Hello Erik,

Just be careful with these. A GHz transistor can oscillate without you even seeing it because none of the equipment is fast enough for that.

Regards, Joerg

Do you really think, it will be a problem in my configuration? The overall gain of my circuit is unity and at the 10mA of DC current used, ft is 2G in tha case of the BFS17 and max.

I could design two versiobs of the board (one with BFS482s and a second with lots of BFS17) but I think temperature drift is much worser with the BFS17-solution. And this drift of DC-level during warm up is the only "feature", which is inconvenient with the russian oscilloscope.

Best regards,

Erik.

Hello Erik,

I have seen unity gain buffers oscillate. That was because some traces weren't really shorts at high frequencies and the buffer developed some 'gain'.

I'd go with the BFS482 if it is better. We have a hard time getting this stuff in the US. EU manufacturers aren't that great with engineering samples. Usually you can't get them and so we often don't use their parts. Temp drift can often be fixed with a tracking diode.

Regards, Joerg